Engine air/fuel control systems responsive to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter are known such as disclosed in U.S. Pat. No. 5,115,639. It is also known to monitor the efficiency of the converter by comparing an output of the upstream sensor to an output of the downstream sensor. In one specific application, comparison of the switching frequency of the downstream to upstream sensor provides an indication of converter efficiency.
The inventors herein have recognized numerous problems with the above approaches. For example, monitoring converter efficiency by comparing upstream EGO sensor to downstream EGO sensor switching frequency is not possible for engines having V-configurations and separate upstream EGO sensors coupled to each cylinder bank. The inventors herein have also recognized that in a conventional switching frequency comparison test, insufficient switching time in the sensors may result in inaccurate efficiency indications. Further, such systems do not examine converter efficiency over the full range of possible exhaust gas flow rates.